182 THE ANATOMY OF VEETEBRATED ANIMALS. 



limbs are tridactyle, the postei-ior didactyle. Menohranclms 

 has tetradactyle feet, while in the other Urodela the anterior 

 limbs are tetradactyle, the posterior pentadactyle. The 

 Batrachia have four digits, with or without a rudiment of 

 another, in the fore-limb, and five in the hind-limb. In 

 the perennibranchiatc Urodela, the cartilages of the carpus 

 and tarsus, which, except in Proteus, present little deviation 

 from the typical number and arrangement (Pig. 11, p. 32), 

 remain unossified ; in the other Urodela, and in the Batrachia, 

 they are for the most part ossified. 



In the Batrachia, the posterior limbs are much longer 

 than the anterior. The radius and the ulna in the fore- 

 Hmb, and the tibia and fibula in the hind-limb, are fused 

 together into one bone. The carpal bones no longer present 

 the typical arrangement ; and, in the tarsus, there are two 

 proximal, gi-eatly elongated, cylindi-ical bones, which take 

 the place of a calcaneum and an astragalus, while the distal 

 series is reduced. 



The limbs of the Labyrinthodonts were feeble in com- 

 parison with the size of the body. In the genera Archego- 

 saurus, Keraterpeton, Urocordylus, Lepterpeton, each foot 

 possessed five digits, and the carpus and tarsus were un- 

 ossified. 



The Amphibia usually possess teeth on the vomers, pre- 

 maxillEe, maxillae, and dentaiy pieces of the mandible, but 

 rarely on the palatine and pterygoid bones. The premaxiUary 

 and vomerine teeth are disposed in concentric semicircles, 

 an an-angement which is very characteristic of the group. 

 In the larvae of the Batrachia, and in Siren, the premaxillae 

 and mandibles are ensheathed m horny beaks, as in the 

 Chelonia and Aves. In addition. Siren has teeth in the 

 vomers, and on the splenial piece of the mandible ; Meno- 

 branchus and Siredon have pterygoid teeth. Many of the 

 Labyi-inthodonts possess palatine teeth. In some Gymno- 

 phiona the mandible has a double row of teeth, and there is 

 an approximation to this structure in the Labyrinthodonts. 



The teeth usually become ankylosed with the adjacent 



